Mortar mix for internal blockwork 7:1

[akist]

Hello everyone,

The builders have worked for a few days now and have completed the blockwork to 1.5m high, both internal walls and inner skin of external walls.

I visited the site and took a note of how much cement and sand they have used so far, and they have used 20 bags of cement and 4.25 bulk bags of sand. I was told that each bulk bag of sand is around 825kg, therefore they have used 3500kg sand and 500kg cement, which makes it a 7:1 mix ratio. I believe this is too weak a mix.

Could anyone please explain this to me in case my calculations are wrong?

 

[jeds]

Mortar mix is done by volume, not weight. Work out what the volume proportions are?

 

[jeds]

ps. I think the average dumpy bag is about 6 bags cement at a ratio of 1:6, which means you'd be looking at, something like, 25 bags cement for 4.25 dumpy bags. Bear in mind that dumpy bags do vary though. Some can be as little as 750kg.

 

[akist]

I asked this morning three merchants (TP, Jewson and IQBuilders) and a delivery driver, they all concurred that the average bulk bag of sand is 825-850kg with Jewsons and TP insisting to more like 850kg. This website has it at 800kg https://source4me.co.uk/calculate_brick_block_mortar.php

It seems cement and building sand are more or less similar density (around 1600kg/m3), so this is the same as measuring by weight.

I spoke to the head-builder, he said one bag of cement is 5 shovels, so they use half a bag cement with 10 shovels sand ( 4 * 2.5 ) and "can't fit more than 10 shovels of sand into the mixer"...

Assuming he is not lying, then the only reasonable explanation is that one 25kg bag of cement is less than 5 of *their* shovels because they do not shovel cement in, they break a bag in half.

 

[stuart45]

Must be a small mixer to only take 10 shovels.

 

[jeds]

Mortar certainly isn't the precise thing it was years ago. I have a 1940 'Hodge' (brickwork for apprentices) where they provide details for the apprentice to make a 'batch box' on site. Those were the days.

What does the mortar look like in the joints? Can you scratch it with a thumb nail?

 

[kingandy2nd]

Perhaps he's stealing your sand for other jobs

 

[kingandy2nd]

Or perhaps he's very generous and had some extra bags of cement from another job he's used on yours first

 

[^woody^]

Never mind counting cement bags, what are the joints like?

 

[akist]

I buy the materials, and I have calculated the spend by what is remaining on site. I do not believe anyone's stealing sand.

If I rub the joints with my finger they do feel dusty/sandy in places. I will have to revisit today and do the finger test again.

The engineer has just said if you can rub the joints with your finger it probably has not got enough cement. On the other hand if the walls are vertical and appear sound then it's probably OK. He very correctly suggests a visit to make a proper judgement.

 

[stuart45]

At a mix of 5/1 I reckon on using between 5 and 6 cement. you do lose some cement on a windy day.

 

[gratefull1]

Are they aircreate blocks, if so I think 6:1 mix is ok. The rule I've heard is that the mortar needs to ne weaker than the blocks, as aircreate are weak (lots of air!) then the mix needs to be weak. I have heard of 9:1 being used.

 

[kingandy2nd]

I buy the materials, and I have calculated the spend by what is remaining on site. I do not believe anyone's stealing sand.

If I rub the joints with my finger they do feel dusty/sandy in places. I will have to revisit today and do the finger test again.

The engineer has just said if you can rub the joints with your finger it probably has not got enough cement. On the other hand if the walls are vertical and appear sound then it's probably OK. He very correctly suggests a visit to make a proper judgement.

If you're buying the materials, what benefit would the builder have by not using the correct mix?

FYI, thermalite blocks are a 6:1 mix when used with plasticiser

 

[akist]

I think it is a matter of incompetency. They assume a bag of cement is 5 shovels, so they cut a bag in half (2.5 shovels), and then add 10 shovels of sand ( 4 * 2.5 = 10 ).

Instead of cutting the cement bag in half, they should empty in on the ground and shovel the necessary quantity in, with the same shovel as the one used for the sand.

 

[kingandy2nd]

I'd be more concerned about the quality of the laying than the mix. Are the walls level, are the beds the right thickness, are the perps aligned, etc? Try pushing a wall down... see if it goes.

Ultimately you've employed these builders matey. If you're not happy, either supervise them doing the mixes, or kick them off site.